Galaxies Under the Cosmic Microscope: a GMOS-North Study of the Lensed Galaxy #289 in A 2218

September 2, 2003

The deflection
of light by the deep gravitational potential of galaxy clusters can be harnessed
to greatly increase the effective collecting area of astronomical telescopes.
Mark Swinbank (Ph.D. student at the University of Durham, U.K.) and his international
team used Gemini's Multi-Object Spectrograph with Integral Field Unit
(GMOS-IFU) on The Frederick C. Gillett Gemini Telescope
(Gemini North) to exploit the rich
galaxy cluster A 2218. They used A 2218 as a magnifying glass to observe
distant background galaxies at a comparable level of detail to galaxies at
redshift z ~ 0.1. This work is part of Mark's Ph. D. project––to
explore the Tully-Fisher relation and chemical abundance gradients in distant
young galaxies.

Figure
1. Optical
image of the massive galaxy cluster A 2218 with the lensed galaxy studied
by Gemini indicated with the yellow arrow. This image was obtained with the
Hubble Space Telescope.

Swinbank et al.,
report on their first result of spatially-resolved [OII]3727 nebular gas
emission from a lensed galaxy at a redshift z=1.034. This corresponds to
an epoch half the present age of the universe. Using a detailed model for
the cluster internal mass distribution, taking into account both visible and
dark matter, they were able to correct the apparent distorted shape of the
galaxy for the lensing by the cluster. The researchers reconstructed the
source morphology and derived the velocity field of the interstellar gas
in this distant object (Figures 2 and 3).

In addition to
distorting images of background objects, gravitational lenses amplify their
intensity several times. In the case of the #289, the overall light magnification
is a factor of 4.92+/-0.15. This leads to a rest-frame absolute I-band magnitude
of MI rest = - 22.4+/-0.2, which makes the #289 galaxy something
of an equivalent of our neighbor M31 undergoing a relatively intense star
forming episode.

Figure
2. On the left is the arc in A 2218 generated by combining three drizzled
frames using HST WFPC2 (I, V and B bands). On the right is the reconstructed
image of the arc corrected for lens amplification.

space

Figure
3. On the left is the GMOS-IFU [OII]3727 emission in shaded blue (scaled for
intensity) with the derived velocity field in the galaxy rest frame superposed.
The lines, in km/s, map regions of same velocity in the rest-frame of the
galaxy. On the right panel, the isovelocity contours are shown with
the image distortion corrected. The background is not color-coded.
Blue is approaching and red is receding. Scales are in arcseconds.

The rotational
circular velocity, corrected for the inclination in the plane of the sky,
is 206+/-18 km/s (see rotation curve in the inset of Figure 4). The galaxy
lies very close to the mean Tully-Fisher relation of present day spirals
(Figure 4). The Tully-Fisher relation is a relationship that exists between
a spiral galaxy's luminosity and its maximum rotation velocity. The behavior
of the relation favors a clear preference for hierarchical formation model
indicated by the apparent small change in the I-band magnitudes over a large
range of redshifts, which is illustrated by arc #289.

Figure
4. The red dot on the left panel plot shows Arc #289 on the rest frame B-Band
Tully-Fisher relation, compared to high redshift (z = 0.83) field galaxies
and other data sets. The inset shows the rotation curve derived from the [OII]3727
line GMOS-IFU observations; horizontal error bars are ~0.7 arcsec, seeing
transformed to source frame. The right is the I-band Tully-Fisher relation
(with Arc #289 as the red dot) from the references indicated in the panel.

Gravitational
optics provides a powerful tool to study the nature of mass assembly, chemistry
and star forming activity in galaxies at early epochs. The methods that have
been developed in this Gemini-based study show the potential of IFU observations
of gravitational lensed galaxies as a means of studying the spatially resolved
properties of high redshift galaxies in a remarkable level of detail.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.